Silica encapsulated lead chromate pigment of the primrose yellow shade

ABSTRACT

A primrose yellow shade pigment of outstanding light stability is prepared. The pigment comprises orthorhombic phase particles of a chemically prereduced lead chromate-lead sulfate solid solution, which particles are encapsulated with a dense amorphous silica coating.

ite mics tent [191 Jackson an. 19, 1974 SILICA ENCAPSULATED LEAD [56]References Cited CHROMATE PIBGMENT OF THE 1 1%. UNITED STATES PATENTSYELLOW SHADE 3,639,133 2/1972 Linton 106/298 5 Inventor: Julius Jackson,westfield NJ. 3,370,971 2/1968 Linton 106/298 3,470,007 9/1969 Linton106/298 [73] Assignee: E. 1. du Pont die Nemours and 2,808,339 10/1957Jackson 106/298 Company, Wilmington, Del. Primary Examiner-Delbert E.Gantz [22] Flled' Sept 1972 Assistant Examiner-J. V. Howard [21] Appl.No.: 292,502

Related U.S. Application Data 5 n h fi i t f H d l ht primrose ye ow s ae plgmen 0 cu s an mg 1g [63] ssg gllg of stability is prepared. Thepigment comprises orthorhombic phase particles of a chemicallyprereduced [52] CL 106/298 106/308 B lead chromate-lead sulfate so1idsolution, which parti- 511 1m. 01 .1 c096 1/20 are encapsulated with adense amrphus Silica 58 Field Of seat-611 106/298, 308 B Wang- 1 Claim,N0 Drawings SILICA ENCAIPSULATED LEAD CHROMATE PIGMENT OF THEIPRIIVIROSE YELLOW SHADE CROSS-REFERENCE TO RELATED APPLICATION Thisapplication is a continuation-in-part of application Ser. No. 206,125filed Dec. 8, 1971 now abandoned.

BACKGROUND OF THE INVENTION In Linton U.S. Pat. No. 3,370,97l there isdescribed the manufacture of lead chromate pigments which have greatlyimproved resistance to discoloration upon contact with acids, alkalies,and soap solutions, and upon exposure to light and heat. These pigmentsare composed of lead chromate particles having deposited on theirsurfaces, as a substantially continuous coating, from about 2 to 40percent by weight of dense, amorphous silica, optionally also, withalumina deposited on the silica. A typical process for the production ofsuch pigments involves slurrying the lead chromate particles in anaqueous medium and depositing amorphous silica on them from an aqueoussodium silicate solution at a pH above 6 and a temperature above 60C.Optionally, then, an aqueous solution of an aluminum-containing compoundmay be added to the resulting suspension of silica-coated lead chromateparticles with continued heating above 60C., whereby alumina isdeposited on the silica coating. Such pigments are particularlyadvantageous in the plastics and paint industries.

The Linton patent exemplifies numerous species of lead chromateparticles that may be used, theseranging from the greenish primroseyellows, which are in the orthorhombic crystal form, to the redishmedium yellows," which are in monoclinic form, as well as to themolybdate oranges and molybdate reds which additionally contain leadmolybdate. Although the temperature resistance of each of these speciesis improved to a very notable extent by the dense, amorphous silicacoating, they are not always as satisfactory as would be desired fromthe standpoint of lightfastness, a property of particular importance infinishes, printing inks, and the like. This is particularly true whenusing the primrose shades of lead chromate pigment.

SUMMARY OF THE INVENTION In accordance with the invention a means hasbeen found for the provision of a primrose yellow shadesilica-encapsulated lead chromate pigment that is characterized byoutstanding light stability characteristics. In particular it has beenfound that the lightfastness of this species of pigment is remarkablyenhanced by the use of a prereduced primrose shade lead chromate pigmentas the base for the silica coating instead of the conventional primroseyellows.

DETAILED DESCRIPTION OF THE INVENTION The term prereduced refers to theknown chemical treatment of the primrose shade lead chromate basepigment to effect a reduction to a lower valence state of the CrO; ionsat the surface of the particles. Such prereduction must take placebefore coating the particles with amorphous silica. In any event thereduction may be accomplished in a number of ways, the most common ofwhich is to admix the lead chromate in aqueous medium with metal ionsreadily convertible to a higher valence state. Antimony compounds, tinand manganese compounds may be used for this purpose, the use of thelatter being illustrated in Jackson U.S. Pat. No. 2,808,339. In the caseof antimony, for example, the reduction that would occur on the surfaceof the lead chromate base particles is as follows:

PbCrO Sb 2H PbCrO Sb H O Apart from the selection of the primrose shade,prereduced base lead chromate particles, other materials involved in thesilica encapsulation process and the processing conditions to beemployed therewith are described in the Linton patent, the disclosure ofwhich is incorporated herein by reference. 5

Three methods of applying the silica coating are illustrated in theexamples of the Linton patent, viz, I) simultaneous but separateaddition of a soluble silicate and a mineral acid, (2) addition of asilicic acid solution freshly prepared by deionizing a sodium silicatesolution with a cation-exchange resin, and (3) addition of sodiumsilicate solution to the pigment slurry, followed by addition ofsulfuric acid.

There are certain critical conditions which must be observed during thetreatment by any of these three methods. To obtain the desired densesilica coatings on the deagglomerated pigment particles, the pH of theslurry at the point of addition of the silica-forming ingredients to themixture must be at least above pH 6.0 and preferably in the range of 9.0to 9.5. Furthermore, the temperature at this same point must be at leastabove 60C. and preferably above C. In many cases it is preferred thatthe temperature be about C. When the pH is allowed to go below about 6.0or the temperature significantly below 60C., there is a pronouncedtendency for the silica to precipitate in an undesired porous, gel-likestructure.

The above-mentioned methods all result in the deposition of at least themajor part of the silica as the desired amorphous continuous film on thesurface of the pigment particles in the slurry. Advantageously thepigment particles will have first been deagglomerated by intense shearso as to give a fluid, easily stirrable starting slurry, e.g., asdisclosed in Linton U.S. Pat. application Ser. No. 83,674 filed Oct. 23,1970, now U.S. Pat.

No. 3,639,133. A convenient starting slurry can contain approximately 25percent pigment, the balance being water, but this concentration is notcritical.

It is common for lead chromate pigments to be finished slightly on theacid side, and when such pigments are reconstituted to form the startingslurry an alkaline treatment is necessary to achieve the desired pH atthe point of subsequent treatment. Such an alkaline treatment can employany convenient alkali, such as ammonium hydroxide or sodium hydroxide,unless the use of a strong alkali such as sodium hydroxide is precludedby the sensitivity thereto of the lead chromate pigment to be coated. Astill more convenient method is to add a small amount of a sodiumsilicate solution; this achieves the desired alkalinity and tends toimprove the dispersion of the pigment particles in the slurry. In such atreatment, care should be exercised not to exceed a pH of about 12.0lest the lead chromate be somewhat solubilized. The sodium silicatesolution used in such an addition as well as in the subsequent treatmentsteps is a commercial product widely available with a SiO Na O ratio ofabout 3.25 and a SiO content of 28.4 percent. Such a product isconvenient but not essential, and other sodium silicate solutions can beused as available. It is desirable, however, that the sodium ion content(Na be kept low, since a high sodium ion content tends to cause gelationof silicic acid.

The above-mentioned methods of forming the silica layer on the surfaceof the lead chromate pigment particles have one feature in common, viz,that the silica is added as active silica. When sodium silicate isacidified, silicic acid is formed, probably initially as orthosilicicacid, Si(OH).,. However, this product tends to polymerize by thereaction of two silanol groups to form a siloxane group Under acidicconditions, this polymerization proceeds rapidly until a predominantproportion of the silanol groups present have been used up in theformation of siloxane bonds. The polymer thus formed has a highmolecular weight and is defined as inactive. Under the conditions ofmoderate alkalinity used in the examples below, there is a low degree ofpolymerization wherein the condensation between silanol groups hasproceeded to only a limited extent leaving the silica in an active" formwhich readily deposits on the surface of the lead chromate particlespresent. It is not intended to imply that there is no polymerization northat the process of polymerization is completely inhibited but, underthe conditions specified, the silica is in a state of low polymerizationand, thus active for a sufficient time to bring about deposition in thedense, amorphous form on the surface of the lead chromate pigmentparticles. This term active silica" has been more precisely definedelsewhere (see Rule US. Pat. No. 2,577,484 for instance), but theconditions specified above are adequate for the purposes of thisinvention.

The quantity of silica to be applied in the pigment treatment can bevaried over a considerable range depending on the intended end use ofthe pigment. For use in extruded hot thermoplastic resins whererelatively high temperatures (200 320C.) are encountered, higheramounts, from to 32 percent of silica, are desirable. There seems to beno great advantage in exceeding about 32 percent, but amounts up toabout 40 percent of the final pigment can be used with some advantage inresistance to the heat treatment, but with some loss in colorproperties. Amounts less than 15 percent show a noticeable improvementover the untreated product but may lack the desired heat resistance. Forother uses, however, such products with lower silica have real value,notably for resistance of coating compositions made therefrom tochemical treatment and to exposure to light. in such applications, aslittle as 2 percent of dense amorphous silica coatings on lead chromatepigments show real improvements over the untreated counterparts. Thus,for purposes of this invention, in its broadest aspects, the denseamorphous silica coating should be in the range of about 2 to 40 percentby weight of the final pigment. When alumina is also present, aconvenient quantity thereof is 0.25 to 2 percent by weight of the finalpigment.

The products of the invention are particularly advantageous when used inthe formation of paints, printing inks, plastics and other productswhere outstanding light stability is desired without a sacrifice inthermal stability.

In a preferred embodiment of the invention, an aqueous slurry of theprereduced primrose shade base lead chromate pigment, to which has beenadded a dilute aqueous solution of sodium silicate, is subjected tointense hydraulic shear, e.g., using a homogenizer, to break up pigmentagglomerates. The homogenized slurry is treated as above described, toinitiate deposition of a continuous silica coating on the particles ofthe pigment. Thereafter an aqueous solution of sodium aluminate oraluminum sulfate is added, the pH is adjusted in the latter case to 9.0to 9.5 and heating is continued, thus altering the composition of thecoating so that it thus contains silica and alumina. Alternatively, inperhaps the most practical mode, the initial deposition of the silicacoating on the pigment particles can be effected by the addition ofdilute sulfuric acid to the suspension of pigment in sodium silicatesolution, and this step then followed by the previously mentionedtreatment to deposit alumina. In either case, isolation of the productinvolves filtration, washing, drying and pulverizing, all of which stepsare conventional. It is understood, of course, that the steps of dryingand grinding may be omitted and the coated pigment product used as anaqueous paste or slurry in subsequent applications.

The following example illustrates this invention in detail. Parts andpercentages as used therein are by weight unless otherwise stated.

EXAMPLE 1 The base pigment employed is a prereduced primrose yellowshade lead chromate available from Harshaw Chemical under thedesignation FR 2100. It is composed of rhombic particles containing 60%PbCrO and 30% PbSO in solid solution with smaller amounts of othercompounds such as basic aluminum silicate and antimony oxide. At thesurface of the pigment particles the CrO, ions have been reduced to alower valence state.

A slurry is prepared of 150 parts of the lead chromate pigment in 1,000parts of water containing 20 parts of sodium silicate solution (Du PontsNo. 9 grade, containing 2829% SiO by analysis; SiO /Na O ratio 3.25).The pH of the slurry is adjusted to 10.6 with 5% NaOH solution and thenpassed once through a homogenizer (Model SMD 15M8TA of Manton-GaulinManufacturing Company) at 5,000 p.s.i. to break up any pigmentagglomerates present in the slurry. The homogenizer is washed with partsof water to remove any residual pigment in the equipment, the washingsbeing added to the original homogenized slurry. The pH of the slurry isreadjusted to 11.5 with 5% NaOH solution, 150 parts water are stirred inand it is then heated to 90C. The following two solutions are introducedsimultaneously with stirring, each at a uniform rate:

1. 1 15 parts of sodium silicate solution, as described above, dilutedto give a volume equivalent to 600 parts of water, and

2. 17.3 parts of 96 percent sulfuric acid diluted with water to give avolume equivalent to 800 parts of water.

Solution 1 is added over 4 hours, solution 2 is added over 5 hours.While maintaining the charge at 90C. the pH is then adjusted to 9.0 with5% NaOH solution. There is then added to the slurry a solution of 9parts of hydrated aluminum sulfate (Al (SO -l8l-l O) in 36 parts ofwater and, after stirring for 2 minutes, a so1ution of 7.5 parts of NaCO in 30 parts of water. After stirring for 2 more minutes there isadded a solution of 11 parts of the same aluminum sulfate in 44 parts ofwater. Stirring is continued for 2 more minutes and the thus-coatedpigment is then isolated in conventional manner by filtration, washingfree of sulfate, and drying at 80C. The dried product is micropulverizedprior to testing.

The resulting pigment is characterized by exceptional light stability.In this regard it is markedly superior to another silica encapsulatedpigment using a conventional primrose shade base particle which has notbeen prereduced.

EXAMPLE 2 In this example a primrose yellow shade lead chromate pigment(Part A) is produced by a technique known in the art (per Example 6 ofthe abovementioned US. Pat. No. 3,370,971) and a portion of it (Part B)is subjected to prereduction using an antimony compound (per US. Pat.No. 2,212,917). Each is then silica-coated and tested for lightfastness.PART A A solution of 331 parts of lead nitrate (Pb(NO in 2,500 parts ofwater at 26C. is adjusted to pH 4.0-4.3 after which a solution of 50parts of sodium carbonate in 400 parts of water is added over a periodof 1 minute. In a separate container 112 parts of sodium dichromate (NaCr O -2H O), 13.8 parts of H 50, (98 percent), 6.2 parts of sodiumsulfate (Na S and 10 parts of aluminum sulfate (Al (SO -18- H O) aredissolved in 2,500 parts of water at 26C. and the pH adjusted to1.8-1.9. This solution is then added in 1 minute to the slurry of leadcarbonate and stirred for 5 minutes, the pH being about 4.0-4.3. Thenthere is added rapidly a solution of 1.7 parts sodium pyrophosphate (NaP O in 40 parts of water, a solution of 4.2 parts sodium carbonate in16.8 parts of water and 29.6 parts sodium silicate (about 28.4% SiOratio of SiO /Na O is about 3.28). A pH of about 6.0-6.2 is recorded.

There is then added at 2 minute intervals, while stirring, a solution of29 parts aluminum sulfate (A1 (SO '18H O) in 116 parts of water, 6 partssodium carbonate in 24 parts water and 1.6 parts sodium pyrophosphate in50 parts of water and the pH adjusted with sodium carbonate to a finalvalue of 5.2-5.5. Thereupon the material is filtered, washed until freeof soluble salts, dried 16 hours at 93C., baked 2 hours at 130C, andpulverized.

PART B The procedure is identical through the first paragraph of Part A.At this point the pH is adjusted to 8.4-8.6 with 5% NaOH solution andthere is added with a 2 minute stir 3 parts antimony oxide dissolved in14.65 parts 37.5 percent hydrochloric acid, 29 parts aluminum sulfate(A1 SO,,) -18H O in 1 16 parts of water, and the pH is adjusted to5.2-5.5 with sodium carbonate. The steps of filtering, washing, andrecovery set forth in the second paragraph of Part A are then followed.

The following procedure is then carried out for preparation of thecorresponding silica-coated pigments:

A slurry is prepared of 150 parts of the lead chromate pigment in 1,000parts of water containing 20 parts of sodium silicate solution (Du PontsNo. 9 grade, containing 28-29% SiO by analysis; SiO /Na O ratio 3.25).The slurry is premixed for 3 minutes and then passed once through ahomogenizer as in Example 1 to break up pigment agglomerates. Thehomogenizer is washed with 200 parts of water to remove any residualpigment in the equipment, the washings being added to the originalhomogenized slurry. The pH of the slurry (10.15 for Part A, 10.2 forPart B) is adjusted to 11.5 with 5% NaOH solution, and it is then heatedto C. and held at that temperature throughout. The following twosolutions are introduced simultaneously with stirring, each at a uniformrate: 5

l. 1 15 parts of sodium silicate solution, as above, di-

luted to give a volume equivalent to 600 parts of water, and

2. 17 parts of 98 percent sulfuric acid diluted with water to give avolume equivalent to 800 parts of water. Solution 1 is added over 4hours, solution 2 is added over 5 hours. While maintaining the charge at90C. the pH is then adjusted to 9.0 with 5% NaOH solution. There is thenadded to the slurry at 1 minute intervals a solution of9 parts ofaluminum sulfate (A1 (SO,) '18- H O) in 36 parts of water, a solution of7.5 parts of Na CO in 30 parts of water and a solution of l 1 parts ofthe same aluminum sulfate in 44 parts of water. The thus-coated pigmentis then isolated in conventional manner by filtration, washing free ofsulfate, and drying at 93C. The dried product is pulverized prior totestmg.

The resulting pigment is tested on a qualitative basis for lightfastnessby dispersing in lithographic varnish (see ASTM Test for Mass Color andTinting Strength D 387-60). Masstone panels are exposed to light by acarbon arc for 24 hours. The pigment of Part B is rated very much betterthan the pigment of Part A. The

thermal stability of the two materials is rated as equivalent (using thetest procedure given in Example 1 of US. Pat. No. 3,639,133 wherein thepigment is dispersed in polystyrene, the composition injection moldedand examined for darkening).

What is claimed is:

l. A lead chromate pigment encapsulated with a coating of 2 to 40 weightpercent of dense amorphous silica and characterized by outstanding lightstability, the base thereof being composed of a primrose yellow shade ofan orthorhombic phase lead chromate-lead sulfate solid solutioncontaining 20-50 weight percent of lead sulfate, the surface of the baseparticles having been prereduced chemically prior to application of saidcoating to convert the chromate of said solid solution to a lowervalence state.

